1. Field of the Invention
This invention relates to an optical apparatus having a lens group capable of accomplishing zooming action.
2. Related Background Art
Numerous optical designs are heretofore known for photo-taking lenses of variable focal length used in video cameras or the like, i.e., so-called zoom lenses. Particularly, a so-called four-group zoom lens in which a first lens group forward in the optic axis is a lens for focus adjustment, a second lens group is a lens group for magnification change, a third lens group is a lens group for correction, the second and third lens group being operatively associated with each other in a predetermined relation to thereby accomplish a zooming operation, and a fourth lens group is a fixed lens group for imaging can be said to be among the most popular zoom lenses. In such four-group zoom lenses, the first lens group for focus adjustment and the second and third lens groups for varying the focal length function quite discretely from one another and therefore, it is not necessary to move the first lens group in response to zooming or move the second lens group for the purpose of focus adjustment. Thus, a lens barrel mechanism could be achieved by a relatively simple construction.
In contrast, there is known a zoom lens of so-called inner focus type in which a third lens group and subsequent lens groups provide a lens group for focus adjustment. In the case of the lens construction of the inner focus type, unlike the aforedescribed four-group zoom lens, the closest distance at which photographing is possible (in-focus is possible) is varied by the focal length when the lens group for focus adjustment is in its most axially forwardly moved position. Especially, there has been an advantage which cannot be achieved by the four-group zoom lens that at the wide end, in-focus becomes possible up to immediately before the lens. However, on the other hand, in such a zoom lens of the inner focus type, the lens group for focus adjustment lies rearwardly of the lens group for magnification change, and this leads to the characteristic that even if the object distance does not vary, the lens group for focus adjustment must be moved by zooming. This in turn has led to the disadvantage that the construction of the lens barrel mechanism becomes very complex. Therefore, such zoom lenses have heretofore been rarely put into practical use. In recent years, however, the development of an automatic focus adjusting device has put into practical use a system whereby the out-of-focus of the focal plane is directly evaluated and the position of the lens group for focus adjustment is controlled on the basis of this information. By the combination of the automatic focus adjusting device of this system and an inner focus lens, it becomes possible to make the position of the lens for focus adjustment a right position even if complicated lens barrel structure is not adopted.
FIGS. 5 to 8 of the accompanying drawings show several examples of the inner focus lens. In the type of FIG. 5, a first group lens 1 is fixed and the position of a second group lens 2 (solid line) is the positioned of the focal length (the wide end) on the wide side and the position of 2' (dots-and-dash line) is the position of the focal length (the telephoto end) on the telephoto side. Also, in this example, as in the conventional four-group zoom lens, a third group lens 3 is operatively associated with the second group lens in a predetermined relation, and the position of 3 (solid line) is the wide end position and the position of 3' (dots-and-dash line) is the telephoto end position. The second and third group lenses, like the lens barrel mechanism construction of the conventional four-group zoom lens, are operatively associated with each other. for example, by a cam ring. The reference numeral 4 designates a lens group for focus adjustment, and this lens group is constructed so as to be variable in position in the direction of the optic axis within a predetermined range as indicated by arrow.
The case of FIG. 6 is a case where the lens group corresponding to 3 in FIG. 5 is absent. Also, in this example, the lens group 4 is divided into a forward lens group 4A and a rearward lens group B, the forward lens group 4A being fixed, and the rearward lens group 4B being provided as a lens group for focus adjustment and being constructed so as to be variable in position in the direction of the optic axis within a predetermined range.
In the example shown in FIG. 7, first and fourth lens groups 1 and 4 are fixed and the position of a second lens group 2 is the wide end position and the position of 2' is the telephoto end position. Also, 3 is a lens for focus adjustment which is constructed so as to be variable in position in the direction of the optic axis within a predetermined range.
In the example shown in FIG. 8, a first group lens 1 is not fixed, but is operatively associated with a second group lens 2 which zooming. Here, 1 and 2 indicate the positions at the wide end, and 1' and 2' indicate the positions at the telephoto end. Also, a lens group for focus adjustment, as in the example shown in FIG. 6, is provided by the rearmost lens group 4B.
FIGS. 9 and 10 of the accompanying drawings show the relation between the position to be assumed by each lens group for focus adjustment in the inner focus lenses of FIGS. 5-8 and the focal length (the two-group lens position), FIG. 9 showing the relation in the case of the lens types of FIG. 6-8, and FIG. 10 showing the relation in the case of the lens type of FIG. 5. In these figures, the zero position on the vertical axis is the position of the lens group for focus adjustment during the telephoto end .infin. focusing.
As is apparent in FIG. 9, in the case of the lens types shown in FIGS. 6-8, the close distance at which photographing is possible (in-focus is possible) at the wide end is 0 m, is about 1 m at the intermediate, and is of the order of 0.6 m at the telephoto end. Also, in the case of the lens type as shown in FIG. 5, the close distance is 0 m at the wide end, and becomes gradually longer and is about 1 m at the telephoto end. The optical systems of FIGS. 6-8, as can be understood from FIG. 9, are set so that when the lens group for zooming is moved from the wide angle end toward the telephoto end, the lens group for focusing may be moved from infinity toward the close distance and from the close distance toward infinity to thereby maintain in-focus.
FIG. 11 of the accompanying drawings shows the basic concept of an example of the automatic focus adjusting device of the aforedescribed type in which the out-of-focus of the focal plane is directly evaluated. In FIG. 11A, the reference numeral 17 designates the picture plane of a video camera or the like, and the reference numeral 18 denotes a distance measuring field therein which is an area for extracting a signal for effecting automatic focus adjustment. The reference numeral 19 designates the contrast pattern of an object to be photographed. FIG. 11B shows signal processing, and the luminance signal to the contrast pattern shown in (a) is as shown in (b). The luminance signal, when differentiated, is as shown in (c), and the absolute value thereof is as shown in (d). Let it be assumed that the height of (e) at which the absolute value has been sampled and held is A. When as shown in FIG. 11C, the position of the lens group for focus adjustment is plotted as the abscissa and the value of A is plotted as the ordinate, there is obtained a mountain-shaped signal and the lens group position B which is the peak is the in-focus lens position.
FIG. 12 of the accompanying drawings is a block construction diagram showing such automatic focus adjusting devices 12 and 13 combined together with the inner focus lens of FIG. 6 taken as an example. The reference numeral 12 designates a sensor, and the reference numeral 13 denotes an AF circuit for detecting the in-focus state by the output of the sensor 12. The reference numeral 14 designates a motor which is a drive source for driving means which makes the lens group 4B for focus adjustment variable in position in the direction of the optic axis.
Actually, however, in the construction as shown in FIG. 12, it is often difficult to normally obtain the in-focus state particularly during zooming. This is attributable to the fact that within the time required for the automatic focus adjusting devices 12, 13 to detect out-of-focus, judge whether this out-of-focus is far focus or near focus, and determine the direction of rotation of the motor 14, only the second group lens for magnification change is moved and deviates from the locus for continuing to be in focus to the inherent object distance shown in FIGS. 9 and 10.
In view of this point, the applicant's U.S. Application Ser. No. 346,630 (filed on May 2, 1989), which issued as U.S. Pat. No. 4,920,369 on Apr. 24, 1990, has proposed a construction in which the interior of the map shown in FIGS. 9 and 10 wherein the abscissa represents the focal length and the ordinate represents the position of the lens group for focus adjustment is divided into a plurality of blocks (such as I, II, . . . ) as shown, for example, in FIG. 13 of the accompanying drawings. The direction and speed of the lens group for focus adjustment to be moved during zooming are determined, for example, from the differentiated value of the locus passing through the substantially central points of the respective blocks and the speed of movement of the second group lens 2, whereby even if the result of the distance measurement by the automatic focus adjusting device is not obtained, driving means for the lens group for zooming and driving means for the lens group for focus adjustment are driven at a time to thereby eliminate out-of-focus during zooming.
In an inner focus lens, when as shown in FIGS. 9 and 10, the lens group for focus adjustment is in its most axially forwardly moved position. The closest distance at which photographing is possible (in-focus is possible) differs depending on the focal length and therefore, if the selection of the focal length is wrong, in-focus cannot be achieved even for an object to which in-focus is possible.
As a solution to this problem, Japanese Laid-Open Patent application No. 60-143310 discloses a system whereby whether the distance to an object judged by an automatic focus adjusting device is closer than the focusable closest distance in that focal length is judged by calculation. If so, a lens group for magnification change is forcibly zoomed toward the wide angle.
Such a construction is relatively easy in a case where as in an automatic focus adjusting device of the so-called deviation detecting type, the lens position which is in focus can be foreseen from the current position of the lens group for focus adjustment, but cannot be realized in a device which can merely discriminate between near focus and far focus or an automatic focus adjusting device which cannot accurately detect the distance to an object.
As another solution to the problem, it would occur to move the lens group for focus adjustment to the close distance end in that focal length during the out-of-focus on the close distance side, and when in-focus cannot be accomplished nevertheless, zoom the lens group for magnification change toward the wide angle.
However, this system would take a long time before in-focus is reached.
As a means for solving this problem as well, according to the applicant's U.S. application Ser. No. 487,276 (filed on Mar. 2, 1990), which was allowed on Dec. 31, 1990, and the issue fee having been paid but the patent number not yet assigned, used is made both of an actuator for driving the lens group for magnification change and an actuator for driving the lens group for focusing to thereby realize the operation of quickly reaching in-focus even when the photographer photographs an object at a distance shorter than the closest possible photographing distance conforming to each focal length.
In this case, more or less magnification change is involved to quickly obtain in-focus, but generally the closest possible photographing distance is several centimeters from immediately in front of the lens. The focal length is on the wide side and in this area, the rate of variation in the focal length occurring with a predetermined amount of movement of the lens group for magnification change is relatively small and therefore, it is considered that the magnification change occurring to quickly obtain in-focus does not bring about a great feeling of physical disorder.
By combining the inner focus lens as described above and a lens position control device together, in-focus can be quickly accomplished for objects from immediately before the lens to .infin. and in-focus can be maintained stably even during the zooming operation.
However, among the inner focus lens types, in the case of the example as shown in FIG. 9 wherein the focal length for which the closest distance at which photographing is possible is the shortest distance is not at the telephoto end (or the wide end) but is an intermediate focal length, some problems arises.
The first problem is that halfway through zooming, the object passes through an out-of-focus state. Even in the case of the characteristic as shown in FIG. 10, if on the shorter distance side than 1 m, zooming is effected from the wide side toward the telephoto side, in-focus cannot be maintained from halfway. In such case, on the telephoto side with this focal length for which in-focus cannot be maintained as the boundary, in-focus cannot be accomplished with any focal length and the characteristic is relatively easy to see. If zooming is effected from the in-focus state toward the wide side, it will be impossible that the object passes through an out-of-focus state halfway. In contrast, in the case of FIG. 9, even if for example, at the telephoto end, in-focus is effected to an object at a distance of 0.6 m, when zooming is effected toward the wide side, an out-of-focus state will occur in the focal length range shown by A in the figure. Accordingly, unless at the start of zooming, the object distance, a distance at which in-focus can be accomplished with the total focal length is out-of-focus will also occur during the zooming from the telephoto side toward the wide side.
The second problem arises in a case where use is made of both the actuator for magnification change and the actuator for focusing according to the aforementioned U.S. application No. 1-51726) and these actuators are driven at a time depending on the situation.
FIG. 14 of the accompanying drawings shows the position of the magnification changing lens on the abscissa in such a manner that the wide end position is 0 and the telephoto end position is 180 (these lens positions are detected by an encoder or the like). Also, on the ordinate, the position of the focusing lens group is shown by 0 to 210 (the position of this lens is likewise detected by some means) In the figure, the positional relation between the two lenses in the case of .infin. in-focus at the focal length of 50 on the abscissa is indicated by a point P.sub.1. When from this state, focusing is to be effected on an object at a distance of 0 m immediately in front of the lens, the time required for focusing can be more shortened by the lens being moved in such a manner as P.sub.1 .fwdarw.P.sub.5 .fwdarw.P.sub.4 .fwdarw.P.sub.3 .fwdarw.P.sub.0 than being moved in such a manner as P.sub.1 .fwdarw.P.sub.2 .fwdarw.P.sub.0. That is, this effect is obtained because during the movement P.sub.5 .fwdarw.P.sub.3, the actuator for magnification change and the actuator for focusing are driven at a time. In the case of FIG. 13, the closest distance for which in-focus is possible at all focal lengths is about 1.0 m. Also, the closest distance for which in-focus is possible at the telephoto end is about 0.5 m. Accordingly, if such movement as previously indicated by P.sub.1 .fwdarw.P.sub.0 effected when from a point P.sub.6 at the telephoto end (in-focus to a distance intermediate of 0.5-1.0 m), the lens is focused on an object at a distance of 0 m immediately in front of the lens, the movement will become such as indicated by P.sub.6 .fwdarw.P.sub.7 .fwdarw.P.sub.8 P.sub.0, and it will be seen that between P.sub.6 .fwdarw.P.sub.8, the in-focus distance halfway of focusing becomes longer than at the starting point of time (the in-focus distance at P.sub.6) Such movement is very unnatural, and when in the combination with an automatic focus adjusting device, out-of-focus becomes more remarkable between P.sub.6 .fwdarw.P.sub.8, it is feared that the judgment of direction will reverse intermediately of P.sub.6 .fwdarw.P.sub.8 and will return to P.sub.6.